System Freezes - Hard Reset Required

Hello everyone,
I have a dilemma with my system. It randomly freezes while I'm using it and requires a hard reset. It does not blue screen, it does not get slow and it does NOT unfreeze no matter how long you allow it to sit. It seems like it freezes more often when I'm running heavy graphics, but it could just be that the load is higher overall.

I have run memtest for days on end with no errors, updated the BIOS, adjusted the voltage settings (a recommendation I found for the motherboard and memory) and tested under multiple operating systems (Windows XP 64-bit, Windows Vista Ultimate 64-bit, Knoppix, Ubuntu) and received the same results.

Here are all the components, currently running on Windows XP Professional 64-bit:

Motherboard: XFX nForce 790i Ultra SLI
Memory: 4x 2GB Patriot 1333MHz DDR3 7-7-7-20
Video: 2x EVGA nVidia GeForce 9600GT 1GB DDR3
CPU: Intel Q9300 2.5 GHz
PSU: Enermax 1000W
HD: 3x 500 GB Western Digital (RAID5)

I've also thrown in extra fans, a Zalman CPU fan/heatsink and a couple DVD drives. The CPU rarely gets above 50 C.

At this point I'm thinking it's either a bad motherboard, two bad video cards or possibly some kind of conflict between the motherboard's RAID and nVidia's software storage controller (which I've disabled, but still shows up). I may be testing the RAID soon (by reinstalling to a single drive) but I'm not sure how to test the other hardware.

I don't know how easily this can be solved, but I would greatly appreciate any support and I'll do my best to answer any questions you have. Thank you!

How hot do your graphics cards get? Are you getting any error messages or blue screens?

Some more stats from BIOS: EDITED WITH UPDATED STATS

MPS: 1.4

FSB - Memory Clock Mode: Linked
FSB - Memory Ratio: Sync Mode
FSB (QDR): 1333 MHz

Memory Timing:
tCL: 7
tRCD: 7
tRP: 7
tRAS: 20
Command Per Clock: 2T

tRRD: 5
tRC: 34
tWR: 12
tWTR: 16
tFAW: 26
tREF: 7.8uS
tRFC: 74

Voltage:

CPU Core: 1.30000
CPU FSB: 1.30V
Memory: 1.750V
nForce SPP: 1.40V
nForce MCP: 1.550V

Cores are set to +00mv, +00mv, +00mv, +00mv

Intel Speedstep is disabled. CPU Thermal control is disabled. C1E Enhanced Halt State is disabled. VT is enabled.

Any other BIOS stats I haven't touched or really looked at.

Edit: Interesting news! Running the 'torture test' with Prime95 locks it up within seconds (tested three times). What do I do with this information? Also, I'm rerunning memtest86+ tonight (for the next 8-12 hours).

memtest is on pass 7 right now (12 hours in). I'm thinking I should let it keep running until after pass 8 before I mess with more settings.. what do you think? edit: pass 9 finished with no issues

I've reset the 'auto' stuff to hard numbers, changed the voltage settings on the different CPU cores to all be +00mv and updated the previous posts with current information. Also, I think 'sync' mode for the timing is 1:1, because the other options are Auto, 5:4 and 3:2. Also, I see options for 'P1' and 'P2' that are currently set to 'Auto' (the other option is Enabled).

Sorry about the delay here.

I've tried a few different things. First, I turned Speed Stepping back on. I've also been running with the case open and an additional floor fan nearby. Neither of these helped.

When I have time later I'll try running with one stick of RAM at a time. My next plan is to back up all my data to an external drive (run dd off a Knoppix CD overnight...) and then try a fresh install to a single disk.